The present invention relates to a lighting device for a discharge tube.
In recent days, a discharge tube (for example, a fluorescent lamp) is more likely to employ a system arranged to convert a DC voltage into a high-frequency AC voltage through the effect of a lighting circuit composed of an inverter and to apply the high-frequency AC voltage into a resonant load circuit containing the discharge tube itself. The resonant load circuit also contains a resonant inductor and a resonant capacitor for setting a resonant frequency. This type of lighting circuit includes an inverter circuit composed of two power semiconductor switching elements connected between a positive and a negative polarities in a half-bridge structure, which is operated to apply the high-frequency AC voltage onto both ends of the resonant load circuit. The waveform of the current flowing through the resonant load circuit (referred to as resonant current) is resonated into a sinusoidal waveform through the effect of the inductor and the capacitor. This resonant current is controlled by changing an operation frequency of the inverter.
As a prior art for stabilizing the driving frequency of a switching element, there has been proposed a stabilizing circuit disclosed in JP-A-8-45685. This circuit provides a half-bridge circuit for supplying an AC voltage to the resonant load circuit containing the discharge lamp and is operated to divide part of the resonant current into a capacitor and a feedback transformer and apply a control signal to the switching element between a high side and a low side of the half-bridge circuit according to a voltage on the secondary side of the feedback transformer. Unlike the normal fluorescent lamp, this prior art discusses the lighting device for an electrodeless fluorescent lamp for generating plasma through magnetic lines of force emitted from an exciting coil. The electrodeless lamp operates to supply high-frequency current of several megahertz to a solenoid type exciting coil and thereby emit magnetic lines of force for generating ions inside of a bulb through the inductive discharge and form those ions as discharge current in a closed loop (plasma) through the effect of electromagnetic coupling applied by the force of magnetization. Mercury vapor in the plasma is excited by the inductive electric field so as to fire ultraviolet rays onto a fluorescent material coated on the inside of the tube on which the ultraviolet rays are converted into visible rays.